In circuit failure analysis, such as Ultra-Large Scale Integration (ULSI) circuit failure analysis, Transmission Electron Microscopes (TEMs) are typical tools that may be used. For preparation of TEM samples, the circuits for testing have to undergo localisation of the site of interest. The circuits typically comprise multi-layered structures on a wafer substrate.
TEM samples are typically prepared so that they may be viewed in either cross-sectional or planar-view orientations (where the orientations are with respect to the circuits).
For cross-sectional view TEM samples, localization of specific areas for TEM analysis is typically achieved with the use of a Scanning Electron Microscope (SEM) in a dual beam Focussed Ion Beam (FIB) system. As illustrated in FIG. 1, the SEM beam column (102) and a tilted FIB column (104) are calibrated and adjusted to “coincide” on a sample at a “coincidental” point (106). Using this technique, the specific area to be localised may be marked out by the SEM beam column (102) while the actual milling of the area to cut out the TEM sample may be conducted with the FIB column (104).
During TEM sample preparation, it is important to avoid or minimise any modification of the sample to be investigated. For example, it has been found that current TEM sample preparation processes have a low success rate in solving failure analysis cases related to small particles (<100 nm), metal/TiN stringer defects, or thin film analysis on wafers. Protective coatings are typically deposited over the site of interest using ion-beam assisted deposition of e.g. Platinum (Pt) to prevent features at the site of interest from being sputtered off during ion-beam milling to cut out the TEM samples. However, it has been found that during the ion-beam assisted deposition of the protective coating itself, damage may already have been done.
While it has been suggested to lower a current of the ion-beam during the deposition of the protective coating, some modification/damage of the site of interest is typically still observed, such as amorphorization.